This invention relates to an electrode substrate suitable for fuel cell, particularly phosphoric acid fuel cell and a process for producing the same.
Phosphoric acid fuel cell is constituted of a maxtrix layer impregnated with a phosphoric acid solution which is an electrolyte, and positive and negative electrodes sandwiching the matrix layer from both surfaces. Electrodes are constituted by forming catalyst layers on the surfaces of electrode substrates which are contacted with the matrix layer.
The electrode substrates to be used for such phosphoric acid fuel cell are demanded to have various electrical, chemical and mechanical characteristics, including high electroconductivity as a matter of course, chemical stability at high temperatures, excellent gas permeability, and also high flexural strength, etc.
More specifically, the electrode substrate for phosphoric acid fuel cell, since it is required to have the function to feed a fuel gas (hydrogen gas, etc.) or oxidative gas (air or oxygen gas) to the catalyst layer adjacent thereto, is first required to have basically high gas permeability. Gas permeability of the electrode substrate is given by porous property of the electrode substrate, namely by the electrode substrate having interconnected pores, and gas permeability performance is determined by the pore size and porosity, etc. If interconnected pores having large pore sizes are formed in the electrode substrate for imrovement of gas permeability of the electrode substrate, the phosphoric acid solution will be readily scattered therethrough, whereby excessive consumption of the solution occurs. On the contrary, if interconnected pores having small pore sizes are formed, the phosphoric acid solution penetrate to the electrode substrate from matrix layer through the capillary phenomenon, whereby its gas permeability may be lowered or the cell performance may be lowered due to shortage of the electrolyte in the matrix layer. Thus, setting of the pore size and porosity within optimum value ranges is one of important matters which influences electrode performances.
Also, a phosphoric acid fuel cell, in order to obtain a power enough to be provided for commercial use, is stacked up with a large number of cell units shaped in flat plate, and therefore the electrode substrate must have high mechanical strength, particularly flexural strength. High mechanical strength is also necessary for performing easily workings of wetproof treatment of the electrode substrate or coating of the catalyst, assembling working of cell units, etc. This demand for mechanical strength becomes greater as the substrate area of the fuel cell becomes larger. Further, for improvement of electroconductivity between the contacted surfaces of the respective cell units, the stacked cell units are clamped with a given compressive force in the stacking direction. Accordingly, the electrode substrate of each unit must have uniform characteristics relative to compression. If the amounts to be compressed when a constant compressive force is permitted to act differ depending on the electrode substrates of the respective units, the characteristics of gas permeability of the electrode substrates of the respective cell units will be also different, whereby variance occurs in cell characteristics between the units, and the heights of the respective units become irregular.
For the electrode substrate of a phosphoric acid fuel cell comprising a large number of cell units stacked up, the surface roughness is also an important problem. More specifically, the electrode of a phosphoric acid fuel cell comprises a catalyst layer consisted of platinum or platinum alloys supported on a carrier coated on the electrode substrate surface, and the coating thickness of the catalyst is generally 50 to 500 .mu.m. Therefore, when the surface roughness of the electrode substrate is great, planar variance of the coated amount of the catalyst is not negligible and power generation irregularity occurs to cause lowering in power generation efficiency. Also, for performing practical power generation, a large number of cell units may be stacked up electrically in series. In this case, since an electroconductive gas separator is sandwiched between the adjacent unit cells, one surface of the electrode substrate is assembled so as to be contacted with the gas separator. If the surface of the electrode substrate is rough, the contact between the substrate and the gas separator becomes insufficient, whereby electrical resistance at the contacted surface is increased to lower power generation efficiency.
As the substrate constituting the electrode of fuel cell as described above, there have been known in the art those comprising short carbon fibers dispersed in random directions within substantially 2-dimensional plane bonded to each other with carbonized or graphitized resin by means of paper making, etc., as disclosed in the literatures such as Japanese Patent Publications Nos. 53-18603 and 53-43920, Japanese Laid-Open Patent Publications Nos. 57-129814, 57-166354 and 60-44963 and others.
All of these substrates of the prior art described in these literatures had one advantage and one shortcoming in gas permeability, mechanical strength, compressive characteristic, etc. subh that some are good in gas permeability but involve a problem in mechanical strength, or on the contrary others are high in mechanical strength but inferior in gas permeability, etc. Thus, no electrode substrate has been obtained, which can sufficiently satisfy the requirements in all of these characteristics.
On the other hand, Japanese Laid-Open Patent Publication No. 58-68881 discloses a method for obtaining an electrode substrate all at once according to molding fabrication of a mixture of chopped carbon fibers and a carbonizable resin without recourse to paper making. Since this method relies on molding fabrication, it is necessary to use choped carbon fibers with extremely short fiber length of, for example, 1 mm or less, and threfore mechanical strength is remarkably inferior as compared with the substrate prepared by the paper making method, whereby there is the problem that handling of the electrode becomes difficult in workings of wetproof treatment of the electrode substrate, coating of the catalyst, assembling of cell units, etc. as described above.